“I want to get an overview of the features available in Windows Phone 8.1”
Watch the first session of the Jump Start. Watch other sessions based on topics as needed.

“I’ve built for Windows 8.1 and I want to build Windows Phone too”
Congratulations, you already can. Windows Phone 8.1 allows Universal application develompent. Basically, everything you’ve already built can move over to Windows Phone. Your XAML will probably be a challenge to make a 1-to-1 correspondence, so you might want to re-work some of your screens to fit the phone form factor. The difficulty level is comparable to building apps across iPhone and iPad.